Mobile communication systems developed to provide the users with voice communication services on the move. With the rapid advance of technologies, the mobile communication systems have evolved to support high speed data communication services as well as the standard voice communication services. As a consequence, the wireless data traffic has increased considerably, resulting in needs of higher data rate.
Typically, the data rate can be increased by increasing the frequency bandwidth or improving frequency utilization efficiency. In the latter case, the current generation communication technologies have almost reached to the theoretical limit of the frequency utilization efficiency, it is difficult to further increase the frequency utilization efficiency through technical improvement. Accordingly, the technology of using wide frequency bandwidth receives attention.
Since it is very difficult to secure broad frequency bandwidth in the frequency band (<5 Ghz) on which the current cellular mobile communication system, there is a need of securing the broad frequency bandwidth in the higher frequency band. Since the frequency band available for broadband communication in the bandwidth over 1 Ghz is limited under the current frequency distribution policy, it is proposed to use the millimeter wave band over 30 Ghz for wireless communication.
However, such a high-frequency band communication has a drawback in that the signal attenuation increases significantly as the propagation distance increases. In detail, as the frequency increases, the propagation pathloss increases and the propagation distance decreases, resulting in reduction of the service coverage. One of the key technologies to mitigate the propagation pathloss and increase the propagation distance is beamforming.